In this type of technical field, mobile communication schemes, which may become successors of the so-called third generation, are being discussed by standardization group 3GPP for W-CDMA (Wideband-Code Division Multiplexing Access) scheme. Particularly, LTE (Long Term Evolution) and further successor mobile communication schemes are being intensively discussed as successors of the W-CDMA scheme, HSDPA (High Speed Downlink Packet Access) scheme, HSUPA (High Speed Uplink Packet Access) scheme and so on.
FIG. 1 illustrates a schematic view of a mobile communication system. The mobile communication system includes a cell 50, user apparatuses 1001, 1002, 1003 residing within the cell 50, a base station apparatus wirelessly communicating to the user apparatuses, an upper node 300 connected to the base station apparatus and a core network 400 connected to the upper node. The upper node 300 may be a radio network controller (RNC), an access gateway (aGW), a mobility management entity (MME) and so on, for example.
In such a mobile communication system, communications are conducted by assigning one or more resource blocks to the user apparatuses in any of uplinks and downlinks. The resource blocks are shared among a large number of user apparatuses within the system. The base station apparatus determines which of the several user apparatuses the resource blocks are assigned for every subframe having a time period such as 1 ms. The subframe may be referred to as a transmission time interval (TTI). The assignment of radio resources is called scheduling. In downlinks, the base station apparatus transmits shared channels to the scheduled user apparatuses in one or more resource blocks. The shared channel may be referred to as a PDSCH (Physical Downlink Shared Channel). In uplinks, the scheduled user apparatuses transmit shared channels to the base station apparatus in one or more resource blocks. The shared channel may be referred to as a PUSCH (Physical Uplink Shared Channel).
If the radio resources are scheduled, it is necessary to signal which of the user apparatuses the shared channel is assigned for every subframe. A downlink control channel for use in the signaling may include a PDCCH (Physical Downlink Control Channel) or a DL-L1/L2 control channel. The PDCCH may include information pieces such as a downlink scheduling grant, an uplink scheduling grant, an ACK/NACK (Acknowledgement/Non-Acknowledgement information) and a transmission power control command bit, for example. See non-patent document 2 in details, for example.
The downlink scheduling grant includes information on downlink shared channels, for example. Specifically, the downlink scheduling grant may includes information pieces such as assignment information of downlink resource blocks, identification of user apparatuses (UE-ID), the number of streams, information on precoding vectors, a data size, a modulation scheme and information on HARQ (Hybrid Automatic Repeat reQuest).
Also, the uplink scheduling grant includes information on uplink shared channels, for example. Specifically, the uplink scheduling grant may information pieces such as assignment information of uplink resources, identification of user apparatuses, a data size, a modulation scheme, uplink transmission power information and demodulation reference signal information in uplink MIMO.
The ACK/NACK indicates whether the PUSCH transmitted in uplinks has to be retransmitted.
In uplinks, user data (normal data signals) and the associated control information are transmitted in the PUSCH. Also separately from the PUSCH, downlink CQI (Channel Quality Indicator), ACK/NACK for the PDSCH and so on are transmitted in a PUCCH (Physical Uplink Control Channel). The CQI is used for scheduling and AMCS (Adaptive Modulation and Coding Scheme) in the PDSCH. In uplinks, a RACH (Random Access Channel), signals indicative of assignment requests of uplink and downlink radio resources may be transmitted if necessary.
On the other hand, since the mobile communication system includes radio links, there arise some types of signal delay that may not be caused in wired systems. The signal delay may be referred to as radio interface delay or air interface delay. From the viewpoint of faster communications, it is necessary to reduce the signal delay as much as possible.
FIG. 2 illustrates details of the air interface delay. As illustrated in FIG. 2, in addition to the air interface delay, channel delay and operations delay in RNC may be caused. However, the channel delay and the operation delay within the RNC can be significantly reduced and are not important to this application, and thus the channel delay and the operation delay are ignored. In general, the air interface delay includes (a) transmission delay, (b) retransmission delay and (c) reception delay. The transmission delay (a) represents a time period from transmission initiation to transmission completion of all signals. For example, in transmission of data equivalent to 1 TTI, a time period equivalent to about 1.5 TTI is required in whole considering delay for transmission operation. The retransmission delay (b) represents delay for retransmission control (HARQ). Suppose that it is defined in the system that if data transmitted in a certain TTI has to be retransmitted, the retransmission is conducted after 6 TTIs. There are cases that the retransmission is needed or not depending on radio transmission states. Supposing that the retransmission is needed at a likelihood of 50%, the delay of about 3 TTIs (=6 TTIs×½) might be caused in average. The reception delay (c) represents a time period required to receive and modulate transmitted data. In reception of data equivalent to 1 TTI, for example, a time period equivalent to about 2 TTIs may be required. Thus, the air interface delay can be estimated to be about 6.5 TTIs in whole. In this manner, the air interface delay is proportional to the TTI. This means that reduction in the TTI period can reduce the air interface delay. For example, if the TTI is shortened from 1.0 ms to 0.5 ms, the above air interface delay may be reduced from 6.5 ms to 3.25 ms.
The relationship between the air interface delay and the TTI is described in non-patent document 2, for example.
On the other hand, as stated above, if different radio resource assignment methods are applied to different subframes, the applied assignment method must be signaled to user apparatuses for each of the subframes. Even if the TTI is shortened, the information amount required for the signaling may not be significantly changed.
The left side in FIG. 3 schematically illustrates a downlink channel arrangement in the case of TTI=1.0 ms. One or more of a large number of (frequency) resource blocks are assigned to certain users. The assignment methods are signaled in a L1/L2 control channel. The right side in FIG. 3 schematically illustrates a downlink channel arrangement in the case of TTI=0.5 ms. As illustrated, the TTI is shortened by half while the transmission frequency of the L1/L2 control channel is doubled. The proportion of the amount of control information per unit time such as 1.0 ms (proportion of overhead) increases for shorter TTI, and accordingly the data throughput decreases. From the viewpoint of the data throughput, it is desirable to make the TTI longer and decrease the overhead proportion per unit time, as illustrated in the left side in FIG. 3.
Non-patent document 1: 3GPP R1-070103, Downlink L1/L2 Control Signaling Channel Structure: Coding
Non-patent document 2: Yoshihisa Kishiyama, Kenichi Higuchi, Hiroyuki Atarashi and Mamoru Sawahashi, “Investigations on Radio Parameter Set for OFDM Radio Access in Evolved UTRA Downlink”, IEICE Tech. Rep., vol. 105, no. 240, RCS2005-72, pp. 49-54, August 2005